Check this out...

[rts]

Saw this and found it quite interesting. Don't know how it works. Must be the blind spot of the eyes or summat...

http://www.ebaumsworld.com/trippyredballs.html

[J.Christie]

Great, however I started to focus on the others and they stayed blurred.

EDIT: Just realised they are supposed to be.

[Dusty Jacket]

I fell off my chair and hurt my hand.

[Spencer For Hire]

I don't know exactly how it works, but I think it's all down to retentive vision. If you look away afterwards at a white piece of paper or wall, you'll still see the same dots, but in green. I guess somehow this retentive image on your retina cancels out the actual one in front of you after a while.